docs: def add_range(self, start, part_len, total_len):
          """
          Add range headers indicating that this a partial response
          """
          content_range = 'bytes {0}-{1}/{2}'.format(start,
                                                     start + part_len - 1,
                                                     total_len)
  
          self.statusline = '206 Partial Content'
          self.replace_header('Content-Range', content_range)
          self.replace_header('Content-Length', str(part_len))
          self.replace_header('Accept-Ranges', 'bytes')
          return self